There is a growing demand for high voltage power conversion for a variety of industrial applications. For example, converters with an output voltage of 3.3-13.8 kV and an output power of 1-100 MW may be used in process control in the mining and mineral, power, water, and metals industries.
Challenges to implement high voltage power conversion come from the operating limits of commercially available semiconductor devices. First, commercially available semiconductor devices may have only limited voltage blocking capability such as up to 6.5 kV and a few thousand amperes. Therefore, a high voltage power converter may combine multiple semiconductor devices in series and/or parallel to reach a high voltage AC output. Second, industrial applications employing power converters typically require the AC outputs of power converters to have low harmonic distortion. AC outputs of power converters are typically pulsed multi-level waveforms. In order to reduce the harmonic distortion, it is desired to increase the frequency of the pulses by increasing the switching frequency of the semiconductor devices. However, semiconductor devices may work up to limited switching frequency. For example, low-voltage devices with blocking voltage of 0.6-1.7 kV may operate up to 20 kHz. However, high-voltage devices with blocking voltage of 3.3-6.5 kV may be switched only at a few hundred to 1000 Hz, which is not much higher than the fundamental frequency of the output voltage and therefore produces poor harmonic distortion.
U.S. Pat. No. 5,625,545 to Hammond discloses a power converter that employs a cascaded H-bridge circuit. An exemplary 3-phase cascaded H-bridge circuit 100 is illustrated in FIG. 1. With reference to FIG. 1, the 3-phase cascaded H-bridge circuit 100 consists of 3n H-bridge power cells 102(1-n), 104(1-n), and 106(1-n). The H-bridge power cells 102(1-n), 104(1-n), and 106(1-n) are identical. Each H-bridge power cell 102(1-n), 104(1-n), and 106(1-n) includes a DC bus capacitor 130 and an H-bridge inverter composed of Insulated Gate Bipolar Transistors (IGBTs) 122 and diodes 124. The H-bridge power cells 102(1-n), 104(1-n), and 106(1-n) are cascaded in series to form 3-phase AC outputs A, B, and C. The DC bus capacitors 130 of the H-bridge power cells 102(1-n), 104(1-n), and 106(1-n) must be isolated from each other and may be supplied from secondary windings of an input multi-winding transformer in order to provide the necessary electrical isolation. In practice, a cascaded H-bridge circuit typically employs low-voltage semiconductor devices that may be switched at a high frequency to reduce harmonic distortion, and uses a great number of H-bridge power cells to output high AC voltages. For example, a commercial 10 kV 3-phase cascaded H-bridge inverter may consist of 27 H-bridge power cells using 1.7 kV IGBTs. It may require an input multi-winding transformer having at least 27 secondary windings, each of which supplies the DC bus of one H-bridge power cell. The size and weight of the input multi-winding transformer may become significant and add complexity and cost to the power converter.
U.S. Pat. No. 5,459,655 to Mori et al. discloses a power converter that employs a multi-level diode clamped circuit. An exemplary single-phase multi-level diode clamped circuit 200 is illustrated in FIG. 2. With reference to FIG. 2, the multi-level diode clamped circuit 200 may include multiple DC capacitors 231, 232, 233, 234, 235, and 236 that are connected in series to form a DC bus. The DC capacitors 231-236 may be identical and therefore divide the DC bus voltage into equal levels, for example, seven levels along the DC bus of the exemplary multi-level diode clamped circuit 200 in FIG. 2. The multi-level diode clamped 200 may include cascaded IGBTs 222 each in parallel with a free-wheeling diode 224, along with cascaded clamped diodes 226. By controlling the IGBTs 222 with appropriate switching signals, the AC output terminal A may be electrically connected to each of the seven discrete voltage levels along the DC bus, and thus generates a 7-level voltage waveform.
A 3-level diode clamped circuit, as shown in FIG. 3, is most practically used, because of the well-known challenge to operate a diode clamped multi-level circuit beyond three levels while still balancing the DC voltages across the DC capacitors. However, a 3-level diode clamped circuit poses other challenges in use. Because its output voltages have only three levels, a 3-level diode clamped circuit provides poor harmonic distortion and needs an immense amount of filters to smooth out output harmonics. Furthermore, a 3-level diode clamped circuit may not be able to generate AC outputs of required amplitudes. For example, using 6.5 kV semiconductor devices, the output of a 3-phase 3-level diode clamped circuit may only reach 4.16 kV. To output higher AC voltages such as 6.6 kV or larger, multiple semiconductor devices may have to be combined in series and switched together as a single device. Semiconductor devices may have non-uniform switching and conduction characteristics because of manufacturing variations. Synchronization of multiple semiconductor devices may have to require special hardware and software designs that add complexity and weakness prone to instability and fault.